It's actually kind of amazing that the US does not have a Mexico policy.
I've posted mine many times, but here's the short version:
Hungary:Croatia = USA:Mexico.
We need a military frontier for so many reasons. Just as the Kingdom of Hungary did with Croatia and the Croats, we should create a 100 or so mile deep dependency south of the existing border. It's southern extent should be chosen based on its purpose: Physical interdiction of contraband, including Mexicans, and security for US citizens living in Texas and the Southwest.
The badasses who run this place will all be former Mexicans, who must learn English and follow orders.
Any current residents of the military frontier who prefer living in what's left of Mexico can leave. Ten years of good quality service in the Frontier Corps buys US citizenship for you and your children.
Mexico is a sovereign country and will not be invaded by the United States. The Mexican government might ask for more assistance, as Colombia did, but that will be at Mexico’s invitation.